Non-volatile semiconductor memory cells permitting charge storage capability are well known in the art. The charges are typically stored in a floating gate to define the states of a memory cell. Typically, the states can be either two levels or more than two levels (for multi-level states storage). Mechanisms such as channel hot electron (CHE), source-side injection (SSI), Fowler-Nordheim tunneling (FN), and Band-to-Band Tunneling (BTBT) can be used to alter the states of such cells in program and/or erase operations.
Ballistic transport of charge carriers is a well-known phenomenon in solid state physics and can be another mechanism for cell operations. Ballistic transport represents charge carriers transporting in an active layer of conductive material (e.g. a semiconductor crystal) without scattering at all (i.e. transporting in a “ballistic” way). The active layer has to be kept thin enough to permit carrier transmission with substantially no scattering. Under these conditions, the transport of carriers resembles that in vacuum, but with the effective mass and group velocity of the carriers in the conductive material. One application of such physics has been proposed in a three terminals device (or “transistor”) in an article by Mead (see Mead, “The Tunnel Emission Amplifier,” Proceedings of the IRE, vol. 48, pp. 359-361, 1960), wherein a tunneling structure having symmetrical energy band structure is proposed. However, there are several fundamental problems associated with the energy band structure of transistor in the noted article when employed for the ballistic transport mechanism. Further, there are fundamental problems when such mechanism and band structure are considered for non-volatile memory operation.
The present invention solves these problems by providing a barrier height engineering concept on energy band structure, by providing a novel method altering barrier heights, by providing charge filter structures, and by providing new memory cell structures.